High-voltage fuse having high speed ratio

ABSTRACT

A high-voltage fuse having a high speed ratio, i.e. a speed ratio of 5-6. The latter is achieved without resorting to multiple pulverulent arc-quenching fillers, by merely structuring the helically wound fusible element. The latter includes axially outer ribbon sections provided with serially related relatively short neck portions having equal cross-sectional areas. The fusible element further includes an axially inner section formed by parallel-connected round silver wire having a length which is a multiple of the length of said short neck portions, a crosssectional area which is less than the cross-sectional area of said short neck portions and a silver-severing overlay of a low fusing point metal arranged adjacent the center thereof.

United States Patent [72] Inventor Frederick]. Kozacke 3,291,943 12/1966 Kazacka 337/295 X South HilnptomNJl. 3,341,674 9/1967 Jacobs 337/290 X 1211 APP 53313 FOREIGN PATENTS 523 a: 2 53;" 439,432 12/1935 Great Britain 337/292 484,408 5/1938 Great Britain. 337/293 [73] 1 072 509 9/1954 France 337/293 Newburyport, Mus.

Primary Examiner-Bernard A. Gilheany Assistant ExaminerF. E. Bell [54] HIGH-VOLTAGE FUSE HAVING man SPEED ArmrnyErwin Salw RATIO 4Cla1 SDra 1 F1 w ABSTRACT: A high-voltage fuse having a high speed ratio, LS. a speed ratio of The [aner is achieved without rgsort. 7/1 337/295 ing to multiple pulverulent arc-quenching fillers. by merely III. structuring (h: wound fusible element The Ialter in. 0' 59, cludes axiany outer ribbon secflons provided re.

295, 295, 276 lated relatively short neck portions having equal cross-sectional areas. The fusible element further includes an axially I56] Rdennm cued inner section formed by parallel-connected round silver wire UNITED STATES PATENTS having a length which is a multiple of the length of said short 1,757,753 5/1930 Thiery etal 337/292 neck portions. a c s-se a a ic is s t e 1,856,701 5/1932 Gerdien 337/290 cross-sectional area of said short neck portions and a silver- 2,939,934 6/1960 Kozacka... 337/159 severing overlay ol'a low fusing point metal arranged adjacent 3,261,952 7/1966 Kozacka 337/295 X the center thereof.

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FREDERICK Jo KOZACKA BY M W/m A/ TTY.

LOG CURRENT AMPS HIGH-VOLTAGE FUSE HAVING HIGH SPEED RATIO BACKGROUND OF INVENTION In the United States of America high-voltage power fuses are rated in accordance with a system referred to as E-rating. The fusible elements of high-voltage power fuses rated 100 E amperes or below melt in 300 seconds at an RMS current within the range of 200 to 240 percent of the continuous current rating of the fuse unit. Regarding power fuses having a rating above 100 E amperes, their fusible elements melt in 600 seconds at an RMS current within the range of 220 to 264 percent of the continuous current rating of the fuse unit. The term melting, as used in this context, means interruption of the metallic current-path the fuse, e. g. by a metallurgical reaction between the silver of which the fusible element is made and a silver-severing overlay metal having a lower fusing point than silver, e.g. tin.

The term speed ratio is an indication of the slope of the time-current characteristic of a fuse. There are two definitions of speed ratio depending upon the E-rating of the fuse. The following definition for speed ratio S applies for fuses rated I E and below:

S minimum melting current at 0.l sec./ minimum melting current at 300 sec.

For fuses rated above I00 E the speed ratio is defined by the following term:

S minimum melting current at 0.1 sec./ minimum melting current at 600 see. It is apparent from the above that the smaller the slope of the timecurrent characteristic of a particular fuse, the higher its speed ratio, and vice versa.

High-voltage power fuses for the protection of transformers must withstand high inrush currents without blowing. To be more specific, such fuses must withstand 12 times the full load current of the transformer for 0.! seconds. Thus the protection of high-voltage transformers calls for high-voltage power fuses having a high-speed ratio, particularly fuses having a speed ratio in the range from 5 to 6.

The design of such fuses is difi'lcult. One of the difficulties is to achieve high-speed ratios. Another difficulty is to achieve the high-speed ratio required without resorting to complex means, e.g. combinations of pulverulent arc-quenching fillers having a small conductivity as, for instance, calcium carbonate, and pulverulent arc-quenching fillers having a high thermal conductivity as, for instance, quartz sand. Another difficulty resides in effectively interrupting small overload currents if their duration is excessive and causes the fuse to blow. All these conditions are superimposed upon the prime condition that the fuse must effectively interrupt major fault currents, or short circuit currents, without generating an inadmissible surge voltage.

Fuses embodying this invention comply with all above desiderata, or requirements.

SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of insulating material and a pair of metallic terminal elements closing the ends of the casing. There is a body of pulverulent arc-quenching filler inside of the casing consisting of quartz sand only, excluding any other pulverulent arc-quenching medium. Helically wound fusible element means inside of the casing and submersed in said filler conductively interconnect said pair of terminal elements. The fusible element means includes a pair of axially outer ribbon sections provided with serially related relatively short neck portions having equal cross-sectional areas. The fusible element means further includes an axially inner section formed by parallel connected round silver wires having a cross-sectional area less than the cross-sectional area of each of said relatively short neck portions, but in excess of 50 percent of the cross-sectional area of each of said relatively short neck portions. Said axially inner section of said fusible element means has a silversevering overlay of a low fusing point metal arranged substantially in the center thereof. The length of the axially inner section is a multiple of the length of each of said relatively short neck portions and sufficient to impart to said fuse a speed ratio of 5 to 6.

BRIEF DESCRIPTION OF DRAWINGS FIG. I is a front elevation of a fuse embodying the present invention with some parts broken away to expose to view the inside of the structure; it

FIG. 2 is a section along II-II ofFIG. 1;

FIG. 3 is a front elevation of a detail of the structure of FIGS. 1 and 2 drawn on a much larger scale than FIGS. 1 and FIG. 4 is a section along lVlV of FIG. 3; and

FIG. 5 is a family of time current characteristics explaining the mode of operation of fuses embodying this invention.

DESCRIPTION OF PREFERRED EMBODIMENT Reference character 1 has been applied to indicate a tubular casing of insulating material, e.g. glass-cloth-melamine. The length of the casing 1 depends primarily on the intended voltage rating of the fuse. The ends of casing I are closed by a pair of metallic terminal elements 2 in the form of plugs pressfitted into the ends of easing I. Plugs 2 may be held in position by steel pins (not shown) projecting through casing 1 into plugs 2. A pair of ferrules or terminal caps 3 is mounted upon the outer surface of easing I and each terminal cap 3 is conductively connected to one of terminal plugs 2. The means for establishing these conductive connections have not been shown in FIG. I. They may consist of screws projecting through the end surfaces of caps 3 into terminal plugs 2, and clamping the end surfaces of caps 3 against the axially outer end surfaces of terminal plugs 2. Casing l is filled with a body of pulverulent arc-quenching filler 4. Filler 4 consists of quartz sand only, excluding any other pulverulent arc-quenching medium. To be more specific. all of casing l is filled with quartz sand and the use of any other kind of pulverulent arcquenching filler such as, for instance, calcium carbonate, or gypsum, is avoided in order to simplify the assembly of the structure as much as possible. In FIG. 1 filler 4 has been deleted in the upper portion of the fuse structure to expose to view the parts which are normally submersed in the pulverulent arc-quenching filler or body of quartz sand 4. Actually the body of quartz sand 4 fills the entire casing 1, Le. the space bounded on the top and bottom by the axially inner end surfaces of terminal plugs 2. The axially inner end surfaces of terminal plugs 2 are provided with a pair of straight grooves 2a intersecting at right angles. Reference character 5 has been applied to indicate four elongated plates of insulating material which is heat resistant and heat-shock resistant. Plates 5 are preferably made of a ceramic material. The longitudinal edges of plates 5 extend parallel to the axis of easing l, and the short end edges of plates 5 project into grooves 20 and are firmly held in position by means of a suitable cement. Thus terminal plugs 2 and plates 5 jointly form a mandrel structure for supporting a helically wound fusible element means. The aforementioned fusible element means includes a pair of axially outer ribbon sections 6 provided with serially related relatively short neck portions 6a having equal cross-sectional areas. These relatively short neck portions are formed by circular perforations in ribbon sections 6. In FIG. 3 reference character 2 has been applied to indicate the length of one of the relatively short neck portions 6a of fuse link sections 6. The length of all other short neck portions is equal to that indicated by reference character e. The axially outer ends of ribbon sections 6 are clamped by means of the heads of screws 10 against the axially inner end surfaces of plugs 2 and also soldered to said axially inner end surfaces. The fusible element means further includes an axially inner section formed by parallel-connected round silver wires 7. The number of wires 7 may be two, or more than two, The axially outer ends of silver wires 7 are spotwelded to the axially inner ends of ribbon sections 6. The spot welds may be covered by solder joints as indicated at 8 in FIGS. 1 and 4. The aggregate cross-sectional area of the two silver wires 7 is less than the cross-sectional area of each of the relatively short neck portion 6a of ribbon sections 6, i.e. is less than the smallest cross-sectional area of ribbon sections 6 as determined by the size of the circular perforations provided therein. On the other hand the aggregate cross-sectional area of wires 7 is 50 percent or more of the cross-sectional area of necks 6a. A silver-severing overlay 9 of a metal having a lower fusing point than silver is arranged substantially in the center of round silver wires 7. The length E of round silver wires 7 is a large multiple of the length e of the short neck portion 6a of ribbon sections 6.

In the structure shown and described there is a one-to-one relation between the length of silver wires 7 and the speed ratio S. Consequently the length E may be expressed in terms of speed ratio 5 rather than in terms of inches. The length E of silver wires 7 required to achieve a speed ratio of 5 to 6 may difier from the length thereof required to generate by backburn the arc voltage needed to interrupt relatively small overload currents. If the former length is less than the latter, wires 7 burn back beyond their ends into ribbon sections 6 and thus interruption of protracted overloads is finally achieved. The length E of ribbon wires 7 to achieve a speed ratio of 5 to 6 may exceed the length required to generate by backburn the arc voltage needed to interrupt relatively small overload currents. in this case not likely to occur at circuit voltages of 5 kv. and exceeding 5 kv. less than the entire length of wires 7 would be vaporized by backburn incident to interruption of relatively small overload currents. The speed ratio of the fuse tends to increase the closer the aggregate cross-sectional area of silver wires 7 comes to the lower limit value of 50 percent of the cross-sectional area of neck portions 60. on the other hand. the aggregate cross-sectional area of silver wires 7 must be sufficiently close and so close to the cross-sectional area of neck portions 60 that on occurrence of major fault currents the latter melt and form series arcs before the axially outer ribbon portions 6 are vaporized by backburn and elongation of the arcs which take the place of silver wires 7. Such a backburn and are elongation condition is unsatisfactory because the arc voltage of a single long arc formed by backburn and elongation of the are which take the place of silver wires 7 would be much less than the aggregate arc voltage of short series arcs of which each is formed at one neck portion 6a. lfi'. t is the melting i". t of neck portions 6a, and

the melting plus the arcing f t of the breaks formed by silver wires 7 from the time of inception ofa major fault current to the time T of melting of neck portions 60. then It is apparent from these considerations that there are limitations in any particular case of reducing the aggregate cross-sectional area of silver wires 7 below the cross-sectional area of neck portions 6a. As a result of these limitations, i.e. as a result of the need of reconciling the condition of utmost decrease of the aggregate cross-sectional area of silver wires 7 in the interest of maximizing the speed ratio to -6 and the condition of coordination of the cross-sectional areas of silver wires 7 and neck portions 6a so as to result in sequential fusion of wires 7 and neck portions 60 and formation of series breaks by the latter. the aggregate cross-sectional area of wires 7 ought gcuernlly to be within the range of 70-80 percent of the cross-sectional area of neck portions 6a. This specific range for the aggregate cross-sectionul area of silver wires 7 applies for high-voltage power fuses intended to be applied for the protection of transformers in average industrial circuits having a voltage rating of5- l5 kv.

On occurrence of major fault currents as caused by bolted short circuits. first silver wires 7 fuse. and thereafter fusion occurs at the short neck portions I60 of ribbon fuse links 6. This sequence of fusion is rfterm ined by the relative sizes of the cross-sectional areas of the necks 6a of ribbon section 6 and of round silvery-vires 7. In addition to the reasons given above sequential fusion of silver wires 7 and necks 6a is also necessary to avoid voltage surges concommitant with blowing of the fuses under major fault current conditions. If the aggregate cross-sectional area of silver wires 7 were not less than the minimum cross sectional area of ribbon sections 60 the temperature gradient from the axially outer ends of round wires 7 to the center thereof where overlay 9 is located would not be sufficiently steep to achieve speed ratios of 5 to 6. An increase of the cross-sectional area of silver wires 7 so as to be equal to that of neck portions 6a would not only result in a drastic decrease of the speed ratio but tend to result in the generation of surge voltages incident to blowing of the fuse on major fault currents.

To optimize the ratio of the aggregate cross-sectional area of silver wires 7 and the cross-sectional area of neck portions 6a, and to optimize the length E of silver wires 7 to obtain the desired speed ratio requires some experimentation. Since this experimentation involves but two parameters, i.e. the ratio of the cross sections of pans 7 and 6a and the length of parts 7. and since these two parameters must lie within the limits set forth above and result in the specific relations and per formance characteristics set forth above. experimental op timization of these two parameters can readily be achieved.

Referring now to FIG. 5 showing three time-current characteristics. time-current characteristic A refers to a fuse substantially as shown in FIGS. 1 and 2 wherein the parallel connected wire sections 7 have been omitted, i.e. wherein the entire fusible element is formed by a ribbon having circular perforations forming serially related relatively short neck portions. Time-current characteristic B refers to a fuse as shown in FIGS. 1 and 2 including a pair of parallel connected silver wire sections 7. The fuse to which time-current characteristic B refers was provided with a pair of silver-severing low fusing point metal overlays situated at points 8 adjacent the end of silver wires 7, but lacked the center silver-severing overlay 9 midway between points 8. The fuse to which time-current curve C refers and which is an embodiment of this invention included all the features of FlGs. 1-4, i.e. it included the silver-severing low fusing point metal overlay 9 substantially midway between points 8. It will be noted that time-current curves A, B and C refer to fuses having the same minimum fusing current.

I claim as my invention:

1. A high-voltage fuse including a. a tubular casing of insulating material;

b. a pair of metallic terminal elements closing the ends of said casing;

c. a body of pulverulent arc-quenching filler inside said casing. said body of filler consisting of quartz sand only excluding any other pulverulent arc-quenching medium; and

d. a helically wound fusible element means inside said casing, submersed in said filler and conductively interconnecting said pair of terminal elements, said fusible elements means including a pair of axially outer ribbon sections provided with serially related relatively short neck portions having equal cross-sectional areas and an axially inner section formed by parallel connected round silver wires having an aggregate cross-sectional area less than the cross-sectional area of each of said relatively short neck portions but in excess of 50 percent of the crosssectional area of each of said relatively short neck portions. said axially inner section of said fusible element means having a silver-severing overlay of a low fusing point metal arranged substantially in the center thereof. and the length of said axially inner section being a multiple of the length of each of said relatively short neck portions and sufficient to impart to said fuse a speed ratio of 5-6.

2. A high-voltage fuse as specified in claim 1 wherein the agpair of terminal elements is formed by a pair of metal plugs inserted into the end of said casing and having grooves in the axially inner end surfaces thereof, and wherein said mandrel structure is formed by a plurality of elongated insulating plates arranged inside said casing parallel to the axis thereof and having axially outer edges inserted into said grooves in said axially inner end surfaces of said pair of terminal plugs. 

1. A high-voltage fuse including a. a tubular casing of insulating material; b. a pair of metallic terminal elements closing the ends of said casing; c. a body of pulverulent arc-quenching filler inside said casing, said body of filler consisting of quartz sand only excluding any other pulverulent arc-quenching medium; and d. a helically wound fusible element means inside said casing, submersed in said filler and conductively interconnecting said pair of terminal elements, said fusible elements means including a pair of axially outer ribbon sections provided with serially related relatively short neck portions having equal cross-sectional areas and an axially inner section formed by parallel connected round silver wires having an aggregate cross-sectional area less than the cross-sectional area of each of said relatively short neck portions but in excess of 50 percent of the cross-sectional area of each of said relatively short neck portions, said axially inner section of said fusible element means having a silver-severing overlay of a low fusing point metal arranged substantially in the center thereof, and the length of said axially inner section being a multiple of the length of each of said relatively short neck portions and sufficient to impart to said fuse a speed ratio of 5-
 6. 2. A high-voltage fuse as specified in claim 1 wherein the aggregate cross-sectional area of said parallel-connected round silver wires is 80 percent to 90 percent of the cross-sectional area of each of said relatively short neck portions.
 3. A high-voltage fuse as specified in claim 2 wherein said fusible element is would around and supported by a mandrel structure arranged in coaxial relation to said casing.
 4. A high-voltage fuse as specified in claim 3 wherein said pair of terminal elements is formed by a pair of metal plugs inserted into the end of said casing and having grooves in the axially inner end surfaces thereof, and wherein said mandrel structure is formed by a plurality of elongated insulating plates arranged inside said casing parallel to the axis thereof and having axially outer edges inserted into said grooves in said axially inner end surfaces of said pair of terminal plugs. 